Temperature stabilization of laser diode for wireless power applications

Abstract

A transmitter for providing eye-safe wireless optical power transmission, comprising a laser diode emitting in the near infra-red region, the wavelength of the beam decreasing with temperature of the laser diode. A temperature sensor is attached to the laser diode, to provide an indication of the junction temperature of the laser diode, and a controller receives an input from the temperature sensor, and reduces the power output of the laser diode according to the temperature measured, in order to limit the laser emission when the temperature measured is below a first predetermined temperature Tl, below which the wavelength of the laser emission is not considered to be eye-safe. When the temperature is below Tl, a heat generator is used to return the wavelength to a safe value. When the temperature rises too much, the laser power is reduced to avoid absorption of the laser power by atmospheric moisture.

Description

[0001] TEMPERATURE STABILIZATION OF LASER DIODE FOR

[0002] WIRELESS POWER APPLICATIONS

[0003] FIELD OF THE INVENTION

[0004] The present disclosure describes technology related to the field of wireless power transmission systems using laser diodes, especially systems for ensuring eye-safe operation of the system.

[0005] BACKGROUND

[0006] Wireless power transmission systems using laser diodes as the source of the transmitted power, often operate in the near infrared region of the spectrum. Since the level of damage to human tissues, and especially the eye, is wavelength dependent, it is important to ensure that the wavelength of the laser diode enables efficient transmission with minimal power loss, and does not leave the regions where it is essentially eye-safe for domestically used power levels.

[0007] The dependence of the wavelength of a laser diode on temperature is well known, but the ramifications of this temperature dependence on eye safety, and the handling of this temperature dependence do not appear to be shown or discussed in any of the known prior art. The prior art references appear to be primarily concerned either with protection of the laser diode itself in the event of its temperature becoming extended above (or below) its normal temperature operating range, or with the wavelength stability of the laser diode when its temperature becoming extended above or below its normal temperature operating range.

[0008] Thus, in US patent application, published as 2022 / 263,350Al for Flexible Management System for Optical Wireless Power Supply”, commonly owned by the present applicant, there is mentioned the use of a controller configured to limit laser emission of the laser diode in response to the temperature measured by a temperature sensor.

[0009] Furthermore, in US patent No. 5,276,697 to J.G.Davis, for “Laser Diode Automatic Power Control Circuit with Means of Protection of the Laser Diode”, there is described a laser imaging system including a laser diode and an automatic power control circuit with a laser diode protection circuit. The protection circuit includes a shunt circuit for shunting power around the laser diode in response to one or more of the following conditions: laser light power over a predetermined maximum value, temperature of the laser diode is below or above a predetermined temperature range, the electrical power supply is below or above predetermined voltage values during power up and power down. Again, this reference is concerned with protecting the laser diode against temperature changes, and nowhere is eye safety mentioned.

[0010] In US patent application, published as US 2020 / 076152 for “Lidar System Operating at 1200- 1400 nm” to J.M. Eichenholtz et al, there are described various methods for heating the laser diode, or for cooling the laser diode using a thermoelectric cooler, in order to maintain the laser diode at a substantially constant operating temperature to reduce the amount of wavelength drift. This reference does mention eye safety, but only in connection with the choice of the wavelength range of operation, whether in the 1200 to 1400 nm range, or the 1400 to 1600 wavelength range. Nowhere does the reference appear to relate the issue of eye safety to wavelength drift of the laser diode.

[0011] The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.

[0012] SUMMARY

[0013] The present disclosure describes new exemplary systems for wireless transmission of laser power, such as are used for providing charging power to portable electronic devices, such as phones and laptop computers. The present disclosure attempts to provide systems and methods that overcome at least some of the disadvantages of prior art laser power transmission systems and methods.

[0014] Laser diodes in the wavelength region between 1305 nm. and 1340nm provide a good balance between human safety and atmospheric transmission. There are many laser diodes available for operation in that spectral region. There is a disadvantage in going to wavelengths longer than 1340 nm, because of the presence of increasingly strong water absorption in a series of absorption peaks from 1350nm to over 1390 nm. Such absorption lines would absorb the beam in its path between the transmitter and a receiver, and hence would reduce the efficiency of power transfer. However, there is also a significant disadvantage in going to wavelengths lower than approximately 1305nm, because of a problem of eye safety at lower wavelengths, as will be explained hereinbelow. The advantage of the wavelength range between 1305nm and 1340nm, and especially its central region, is that for a beam power of less than a few watts, and for a system which will turn the beam off within a short predetermined time, because of the reasonably low water absorption level, impingement of the beam on the skin or on the eye tissues does not lead to meaningful tissue damage. This arises since the low water absorption level in that wavelength region, leads to the power being absorbed over a large depth of tissue, and hence the beam energy is dissipated over a large volume of the tissue. These criterion are valid at the power levels used for domestic laser power transmission, such as for charging phones or laptop computers. Consequently, the laser wavelength used in the current application for laser power charging is around the region of 1320nm, typically between 1305nm and 1340nm.

[0015] The “eye-safesf ’ near-infrared wavelength, according to currently accepted safety standards, is above 1400nm, since the risk of eye damage drops exponentially above 1250nm and out to 1400nm, while the smaller risk of damage to the skin does not change much. Consequently, the impingement of the beam can be felt both on the skin or the eye, before damage may be caused to the eye. On the other hand, it is disadvantageous to use a wavelength at above approximately 1340 nm, because of the increased atmospheric attenuation by water vapor. The systems of the current disclosure operate at 1320 nm or even slightly shorter wavelengths, in order to allow less water absorption by atmospheric water and hence a more efficient power transfer system.

[0016] The near infra-red region, around 1320nm, thus sits in a compromise spectral region between a clear window of low moisture absorption, and a region of high moisture absorption. However, that clear window of low moisture absorption, is potentially dangerous to the tissues of the eye, since the low level of the beam absorption by the water of the eye tissues, does not provide the user, or any other human in the vicinity, with thermal warning that the beam is impinging on his / her eye. On the other hand, the highly absorbing lines at and above the higher end of that region, while reasonably eye safe, will strongly absorb the transmitted beam, thereby reducing transmission efficiency.

[0017] Since laser diodes generally shift their wavelength when their temperature changes, and especially diode lasers in this NIR region, the shift being up to 0.6nm / degree C shift, stabilizing the diode temperature is crucial to safe and efficient operation of such an optical power transmission system.

[0018] The present disclosure describes laser diode wireless power transmission systems and methods, which overcome the problem of stabilization of the laser diode wavelength at the optimum wavelength, by maintaining thermal stability of the laser diode junction to maintain eye-safety criteria. The eye-safety criteria may be those mandated by any safety regulations applicable in the region where the system is intended to operate. The system achieves this by closely monitoring the temperature of the laser diode junction, or of a heatsink attached to the laser diode, and adjusting the power output of the laser diode according to an algorithm which ensures that, should the beam wavelength shift to a lower wavelength involving potential danger to the eye, the system reduces or terminates the level of laser power transmitted. Thus, the system uses manipulation of the temperature of the laser diode junction in order to maintain an eye safe laser wavelength, yet without entering an inefficient transmission region. The system may warm up the laser diode by a heat producing element, such as a diode, and may cool it down by using a cooling mechanism such as a fan, or a Peltier diode, or a cooling heat sink.

[0019] When the temperature of the laser diode rises excessively, the laser power is reduced or even switched off, both in order to avoid damage to the laser diode, and also to keep the laser wavelength out of the high water absorption region, until the temperature falls again so that lasing may occur in the optimum safe region. Additionally, when the temperature of the laser diode falls, causing the wavelength of the laser emission also to fall towards the region of higher risk to the retina of the eye, the laser power is then also either attenuated or switched off to avoid eye damage. However, in order to maintain continuous laser power transmission, rather than repeated increasing and decreasing or even ceasing and restarting the power transmission, a complex thermal control system is utilized in the presently described systems, such that utilization is optimized. The laser power transmission systems thus become more universally useable, such as for instance, for powering devices which do not have a built-in battery. Thus, when the temperature of the laser diode junction decreases to lower levels, including the status at system start-up, when the laser diode will typically be colder than during operation, the system may activate a heat generating unit, to raise the temperature of the laser diode, and hence to raise the wavelength of the laser emission above the more dangerous region to the eye.

[0020] Conversely, when the temperature of the laser diode junction increases, especially when the system is operating in a high power mode, or in a warm environment, or when poor ventilation conditions exist, and the wavelength of the laser emission rises towards the higher water absorption spectral region, the system may either reduce the laser output power somewhat, or even implement a cooling system to forcibly cool the laser diode junction, so that the wavelength of the laser emission decreases again. Thus, the presently described systems differ from conventional prior art diode laser temperature stabilization systems, by the criteria used in the stabilization algorithm. Prior art stabilization systems usually use a control algorithm intended only to prevent undue rise in the laser diode temperature, to prevent damage to the laser diode, while the presently described system uses a control algorithm intended also to prevent undue fall in the laser diode temperature, in order to avoid emitting potentially eye- dangerous lasing wavelengths.

[0021] There is thus provided in accordance with an exemplary implementation of the devices described in this disclosure, a transmitter for providing eye-safe wireless optical power transmission, comprising:

[0022] (i) a laser diode configured to emit a beam in the near infra-red region, the wavelength of the beam decreasing with decrease of the temperature of the junction of the laser diode;

[0023] (ii) a temperature sensor attached to the laser diode, such that it provides an indication of the junction temperature of the laser diode; and

[0024] (iii) a controller, receiving an input from the temperature sensor, and adapted to control the power output of the laser diode in accordance with the temperature measured, wherein the controller is configured to limit laser emission of the laser diode when the temperature measured by the temperature sensor is below a first predetermined temperature Tl, below which the wavelength of the laser emission is not considered to be eye-safe.

[0025] In such a transmitter, the limitation of the laser emission may comprise either a reduction of beam power to an eye-safe level, or cessation of lasing. Furthermore, when the temperature measured by the temperature sensor is below the first predetermined temperature Tl, the controller should be further configured to enable activation of heat generation to increase the temperature measured to a second predetermined temperature T2 above the first predetermined temperature. To accomplish that, the transmitter should also comprise a heat generator to provide the heat generation, or alternatively, the heat generation may be achieved by internal heating by electronic activity of digital circuitry associate with the transmitter.

[0026] In any of the above described transmitters, the first predetermined temperature may correspond to a temperature of the laser junction at which the emitted laser beam has a wavelength less than the wavelength at which the time taken for sensory impact of the impingement of the laser beam on a tissue of human to be felt, is considered to be longer than the time taken for the laser beam to cause damage to the eye of the subject. Alternatively, the first predetermined temperature may correspond to a temperature of the laser junction at which the emitted laser beam has a wavelength less than a wavelength at which it is determined that the time taken from exposure to the laser beam until the beam is felt on a tissue of a human, is longer than the maximum time duration during which it is considered safe to expose an eye of the human to the laser beam.

[0027] In any of the above described transmitters, the controller may be configured to operate the transmitter at its high power mode only after a predetermined time during which it has operated in a low power mode.

[0028] Additionally, the controller may be further configured to reduce the power output of the laser diode to a predetermined reduced level, when the temperature reported by the temperature sensor rises above a third predetermined temperature T3. This third predetermined temperature should correspond to a temperature of the laser junction, at which the emitted laser beam has a wavelength above which the beam has an absorption by water vapor greater than a predetermined level. The predetermined level of beam absorption causes a reduction in the transmission efficiency of the laser beam greater than a predetermined level.

[0029] There is further provided, according to yet another implementation of the present disclosure, a transmitter as described above, further comprising a cooling system, and wherein the controller is configured that when the temperature reported by the temperature sensor rises above a third predetermined temperature T3, the cooling system is activated to reduce the temperature of the laser diode to at least below the third predetermined temperature. The third predetermined temperature may correspond to a wavelength of the laser above which it is determined that absorption of the laser beam by water vapor in the transmitted beam path is such as to cause a reduction in transmission efficiency of the beam to a receiver, greater than a predetermined level.

[0030] Further implementations of transmitters described in the present disclosure may be such that during operation in the high power mode, the controller is configured to terminate lasing when a fall of temperature at greater than a predetermined rate, is detected, to below a fourth predetermined temperature T4, the fourth temperature being less than the first predetermined temperature. Alternatively, during operation in the high power mode, the controller may be configured to reduce the system operation to the low power mode when a fall to below a fourth predetermined temperature T4 is detected at less than a predetermined rate, the fourth temperature being less than the first predetermined temperature.

[0031] Finally, in any of the above described transmitters, the controller may be configured to limit laser emission only when there is a loss of power exceeding a predetermined level, between the power of the transmitted laser beam and the power of the laser beam received at a remote location.

[0032] According to yet further implementations of the present disclosure, there is further provided a method for providing eye-safe transmission of laser power to at least one receiver, comprising the steps of:

[0033] (i) providing a laser power transmitter having a laser diode configured to emit a beam in the near infra-red region, the wavelength of the beam decreasing with decrease of the temperature of the laser diode;

[0034] (ii) using a temperature sensor attached to the laser diode to measure a temperature providing an indication of the junction temperature of the laser diode;

[0035] (iii) controlling the power output of the laser diode in accordance with the temperature measured; and

[0036] (iv) limiting emission of the laser diode when the measured temperature is below a first predetermined temperature Tl, below which the wavelength of the laser emission is not considered to be eye-safe.

[0037] In such a method, the limitation of the laser emission may comprise either a reduction of beam power to an eye-safe level, or cessation of lasing. Furthermore, when the temperature measured by the temperature sensor is below the first predetermined temperature Tl, heat generation may be activated to increase the temperature measured to a second predetermined temperature T2 above the first predetermined temperature. That can be accomplished by use of a heat generator to provide the heat generation, or alternatively, the heat generation may be achieved by internal heating by electronic activity of digital circuitry associated with the transmitter.

[0038] In any of the above described transmitters, the first predetermined temperature Tl may correspond to a temperature of the laser junction at which the emitted laser beam has a wavelength less than the wavelength at which the time taken for sensory impact of the impingement of the laser beam on a tissue of human to be felt, is considered to be longer than the time taken for the laser beam to cause damage to the eye of the subject. Alternatively, the first predetermined temperature may correspond to a temperature of the laser junction at which the emitted laser beam has a wavelength less than a wavelength at which it is determined that the time taken from exposure to the laser beam until the beam is felt on a tissue of a human, is longer than the maximum time duration during which it is considered safe to expose an eye of the human to the laser beam. In any of the above described methods, the controller may be configured to operate the transmitter at its high power mode only after a predetermined time during which it has operated in a low power mode.

[0039] Additionally, the controller may be further configured to reduce the power output of the laser diode to a predetermined reduced level, when the temperature reported by the temperature sensor rises above a third predetermined temperature T3. This third predetermined temperature should correspond to a temperature of the laser junction, at which the emitted laser beam has a wavelength above which the beam has an absorption by water vapor greater than a predetermined level. This predetermined level of beam absorption causes a reduction in the transmission efficiency of the laser beam greater than a predetermined level.

[0040] There is further provided, according to yet another implementation of the present disclosure, a method as described above, further comprising activating a cooling system to reduce the temperature of the laser diode to at least below a third predetermined temperature T3, when the temperature reported by the temperature sensor rises above the third predetermined temperature. The third predetermined temperature may correspond to a wavelength of the laser above which it is determined that absorption of the laser beam by water vapor in the transmitted beam path is such as to cause a reduction in transmission efficiency of the beam to a receiver, greater than a predetermined level.

[0041] Further implementations of methods described in the present disclosure may be that during operation in the high power mode, lasing is terminated when a fall of temperature at greater than a predetermined rate is detected, to below a fourth predetermined temperature T4, the fourth temperature being less than the first predetermined temperature. Alternatively, during operation in the high power mode, the method reduces the system operation to the low power mode when a fall to below a fourth predetermined temperature T4 is detected at less than a predetermined rate, the fourth temperature being less than the first predetermined temperature.

[0042] Finally, in any of the above described methods, laser emission may be limited by the controller only when there is a loss of power exceeding a predetermined level, between the power of the transmitted laser beam and the power of the laser beam received at a remote location.

[0043] In addition to the above described method for providing eye-safe transmission of laser power to at least one receiver, there are further provided additional aspects of that method, incorporating the various additional elements of the transmitters for wireless power transmission, as described hereinabove, and as variously claimed. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

[0045] Fig.l shows schematically, a block diagram of an exemplary temperature stabilized laser system of the type described in this disclosure to ensure eye-safe operation;

[0046] Fig. 2 is a graph of the percentage absorption of light in water vapor as a function of the wavelength of the light over the visible and near infrared regions of the spectrum; and

[0047] Fig. 3 illustrates an exemplary flow chart showing one method by which the control system of the presently described system operates from start-up, while maintaining safe operation of the system, based on measurements and stabilization of the laser diode temperature.

[0048] DETAILED DESCRIPTION

[0049] Reference is first made to Fig. 1, which illustrated schematically, a block diagram of an exemplary laser system of the type described in this disclosure. Laser diode 101 which typically emits more than 200mW of optical power, has a bandgap between 0.8eV and 1.1 eV, corresponding to lasing transitions of between 1195nm and 1540nm. The particular laser diodes used in the systems of the present application are chosen such that they lase at room temperature, at a wavelength between 1305nm and 1340nm. Laser driver 104 supplies laser diode 101 with at least 0.7A of current, causing laser diode 101 to emit at least 200mW of heat, which causes laser diode 101 to heat up, generally to above room temperature. Most of this heat is typically extracted by a heat sink 102, usually air cooled, which is thermally coupled to the laser diode 101, but an active cooling system 109 can also be added to assist in the cooling of the system. Nevertheless the temperature of laser diode 101 generally rises during higher power operation, and its temperature may be monitored by means of temperature sensor 103. Temperature sensor 103 can measure the temperature of the heat sink 102, or of another point in thermal contact with the junction of laser diode 101 and having a known relationship to the junction temperature. The temperature measured is generally not necessarily the same as the laser junction temperature, the heatsink temperature being typically lower than the junction temperature by a number of degrees. The temperature rise of the diode is not instantaneous and it may take even some minutes until it reaches its operating temperature, at which the control function of the presently described system is intended to maintain an equilibrium temperature position.

[0050] The problem of potential overheating of the junction as the temperature of the laser diode 101 rises, is an issue which is well known and is managed in previously available systems. However, another issue that is considered in the present disclosure, is the dependence of the emitted laser wavelength on the junction temperature. When the temperature of the laser diode rises, its bandgap falls, and the wavelength of the laser light emitted thus increases. That situation is acceptable, so long as it does not allow the laser diode temperature to exceed the temperature at which the wavelength reaches values such as 1350nm, 1360nm, 1380nm or 1390nm, where water vapor absorption peaks are centered, reducing the efficiency of power transmission. To illustrate this, reference is made to Fig. 2, which is a graph of the percentage absorption of light in water vapor as a function of the wavelength of the light. The strong absorption peaks at 1350nm, 1360nm, 1380nm and 1390nm are clearly shown on the graph. Therefore, in order to keep transmission efficiency from falling, the laser diode should be prevented from emitting in the range of these wavelengths.

[0051] However, looking at the other end of the laser diode’s operating temperature, a real safety problem is encountered when the temperature of the laser diode 101 drops, and the bandgap becomes larger, resulting in shorter wavelength emissions. This problem becomes significant when the laser diode junction temperature drops down to or below a first temperature threshold T1 which supports lasing at wavelengths such as any of 1310nm, 1309nm, 1307nm, 1305.7nm, 1305.2 nm or even 1305nm. The shorter the wavelengths chosen for operating the system before the need to reduce or terminate the laser power emitted, the less the water absorption of the beam, and because of the lower protection provided for the retina, the greater the danger of eye-damage. Consequently, the lower the wavelengths used in the above selection of wavelengths, the shorter the allowed exposure to the beam, with 1307nm being a practical limit, considering the response time required of the system to prevent damage from a beam having a practical power level for the intended use. That means that for a system with a lower emitted power level, the cut-off wavelength of the diode used before reaching a region which is no longer considered eye safe, can be selected from the lower end of the above mentioned range of wavelengths, such as 1305 nm. Above these wavelengths, the level of water absorption in the eye tissues protects the retina from damage by the laser for a long enough time to allow a human to feel a laser beam falling on their eye or face, and to move away from the beam. Although the beam is invisible, it can still be felt, and thereby provides warning of its presence. This may be termed a body-safe mechanism. However, in this emitted wavelength region and below, water absorption by the eye tissues, which consists mostly of water, falls, such that as the eye’s lens focuses the laser onto a small spot at the retina, the intensity level could damage the retina. Thus the sensory “early warning system” is less effective at these wavelengths, exactly in the region where the damage mechanism to the retina is more dangerous and occurs faster. As the wavelength drops to lower and lower wavelengths it eventually reaches a level below the sensory threshold of many humans. Consequently, eye damage can then result without the subject even being aware of the presence of the laser beam. As such, there is a significantly increased risk of eye damage as the laser diode temperature decreases.

[0052] The above described effects of the laser beam on the human tissue are, to a significant extent, human-dependent. When the wavelength of the laser diode drops into the region where the beam begins to be dangerous to the eye, the time it takes before a beam of specific power density is felt on the skin may become longer than the time duration during which it is safe to expose the eye to that beam. Data for “feeling the beam” based on specific wavelengths is humanal and is considered to be “less scientific”. However a good estimation may be performed by assuming that since temperatures above 42°c are considered to entail a risk of tissue damage, those are the temperatures that may be felt by the body. Even if felt, the time for the body to react and prevent impingement of the laser beam on the eye, is limited by the eye’s ability to blink, which is one of the fastest actions the human body can perform, and takes about 0.125 second.

[0053] To compensate for the above described phenomena, temperature sensor 103 inputs the temperature measured on the laser diode, to a controller 105, and from the level of that temperature, the controller 105 determines the laser wavelength associated with that laser diode temperature, to determine whether it is safely above any wavelength indicating potential danger to the eye, or at least verifies that the temperature is above an acceptable limit, such that the laser wavelength does not implicate a significant danger to the eye. When the temperature of the laser junction falls to or below a level associated with the wavelengths at which it is accepted that eye damage could result before the human becomes aware of the laser impingement, those wavelengths being known as the wavelengths of the predetermined first threshold, controller 105 is programmed to turn on heat generator 108, which is thermally coupled to either the laser diode 101 or the heatsink 102. This heat generation is intended to maintain laser safety, since the emitted wavelength is now more eye dangerous. The controller also precludes the laser from emitting at full power when the low temperature is detected. This status is maintained until the temperature reported by sensor 103 indicates an increase to a level at which the laser emits a wavelength sufficiently high, that it provides the sensory safety afforded, as a result of the human being able to feel the impingement of the beam, and to move away therefrom.

[0054] During operation, a signal from the sensor 103 representing the temperature of the laser diode is continuously input to the controller 105, and, should the temperature drop to, or below, the first temperature threshold Tl, indicative of laser operation at any of the selected wavelengths mentioned above, 1310nm, 1309nm, 1307nm, 1305.7nm, 1305.2 nm or 1305nm, the controller instructs the system to adopt either the predetermined safety system parameters, such as operation at a low power, or to completely switch-off of the laser emission. The temperature indicated by the temperature sensor is typically dependent both on laser power and on laser operation time since start-up,

[0055] In addition, when the laser diode temperature drops to, or below, the first temperature threshold Tl which enables emission of the wavelengths 1310nm, 1309nm, 1307nm, 1305.7nm, 1305.2 nm or 1305nm, then controller 105 may be programmed to actuate heat generator 108 to increase the temperature of the laser junction, or it may provide heating of the laser diode junction in an alternative indirect manner, without applying direct heat from a heat generator 108. Thus, in alternative implementations of these systems, the heat generator 108 may not be a dedicated heating element, but instead, the heat may be generated within the logic and control chips of the device or by any other power generating device in the system, such as from intensive calculations being performed in controller 105, or high activity in any other system component which may generate heat. The high digital activity performed, causing heat generation, may be routine system activity of the digital circuitry, such as increasing screen brightness or transmitting data over a Wi-Fi communication link, or it may be the performance of external functions unrelated to the operation of the system itself. The level of the heat generation can be varied according to the wavelength at which the laser diode is lasing. Thus, for lasing at 1320 nm, a lower level of heat generation is generally necessary than for lasing at 1310nm, since 1310nm is closer than 1320nm to the potentially eye-damaging wavelengths arising from temperatures at or below Tl. On the other hand, should the temperature reported by sensor 103 be higher than a predetermined high threshold temperature, equivalent to wavelengths of 1340nm, 1350nm, 1380nm, or higher, then controller 105 may be programmed to cause laser driver 104 to reduce its output, or to terminate the lasing completely, or to turn on a cooling system 109. In the alternate dissipation based mode of cooling, the controller may reduce the overall heat generation of the system such as by turning on a power saving mode for various components, stopping less critical actions or reducing the duty cycle of other operations, such as slowing down calculation speed if possible, or reducing search performance.

[0056] The beam emitted by laser diode 101 is collimated by optical system 106 and its power is measured by power meter 107, and the power level measured is input to controller 105. The controller can then also determine if there is an obstruction in the path of the laser beam between the transmitter and a receiver intended to receive the transmitted power, by noting whether the difference between the transmitted power and the power received by the receiver is significantly more than the environmental absorption expected over the same distance. Such an obstruction could be the result of a human inserting a body part into the beam. Should that body part include the human’s eye, then damage may be caused to the eye if the human does not move out of the beam sufficiently quickly. Alternatively, the system should have a safety system for curtailing the beam under those circumstances. The knowledge of an obstruction in the beam path is important in the present system, since, if it is determined that there is no human body part obstructing the beam, it enables the controller to allow beam transmission to continue at its high power mode, even if the laser diode temperature is at or below the first temperature threshold Tl, and there is no need to reduce or even turn off the beam because of the potentially eye-harmful low laser diode temperature. However, since complete knowledge is practically unachievable, it would be considered wise to program the reduction of power when the temperature is at or below Tl, even when no indication of human presence in the beam is received.

[0057] This step of causing laser driver 104 to reduce power or to terminate power, in response to detecting low laser junction temperature, is an important feature of the systems and methods described in this disclosure, since for laser emission at wavelengths at or less than these wavelengths, the sensory feedback from the laser may be slower than the damage mechanism to the eye, and hence, the perceptive safety element may be lost. Another important feature of the current invention is that upon startup, if the temperature sensed by temperature sensor 103 is too low, below a second threshold T2 corresponding to wavelengths even shorter than the predetermined first threshold, and enabling lasing down to 1307nm, 1305.7nm, 1305.2 nm 1305nm, 1300nm, or 1298nm, then controller 105 is configured to cause heat generator 108 to increase the temperature and also to cause laser driver 104 to limit or delay the operation of laser diode 101 at high power, if at all allowed to emit.

[0058] Another feature of the currently described systems may be necessary because of the existence of a temperature difference between heatsink 102 and junction of the laser diode 101. The sign of that difference is dependent on the direction of heat flow. Typically, when more heat is generated by laser diode 101 than the heat generated by heat generator 108, heat flows from the laser diode out to the heatsink, and the temperature sensor thus indicates a temperature lower than that of the laser diode junction. Since the emitted wavelength is determined by the laser diode junction temperature, the threshold temperature setting for enabling laser operation, as measured by temperature sensor 103, should be set lower than the threshold temperature used when more heat is generated by heat generator 108 than the heat generated by laser diode 101, the latter implying a cooler laser diode junction than is indicated by the temperature sensor 103.

[0059] The controller 105 should incorporate at least a processor running an algorithm instructing the operation of the controller according to the various information inputs required for operating the system, a memory adapted to store data and any algorithmic instruction set, and an output or communication channel port for outputting operational results of the operation of the system, and for inputting user instructions.

[0060] Although the above described safety features relate to the temperature Tl, below which the laser diode will be emitting a wavelength which is no longer considered to be eye safe, it is known that different samples of a laser diode of the same model, or even of the same manufacturing batch, may have slightly differing wavelength correlations to the temperature of the junction. This may present a problem, since the critical feature which determines whether the laser beam may be damaging to the eye, is the wavelength of the beam and not the temperature of the junction of the laser diode. Although the spread in wavelength which a specific junction temperature Tl generates is small, in view of the importance of ensuring eye safety, a method must be found in order to take account of these minor deviations of laser diodes from sample to sample, even from the same manufacturing batch. The one temperature feature which remains constant is the rate of change of wavelength with change in temperature, dX / dT, since this is an intrinsic property of the junction material, and not of the conditions of the manufacturing process. Therefore, according to an improvement to the accuracy of the determination of when the system temperature falls to a level T1 at which eye safety is compromised, each system may undergo a preliminary calibration procedure, in which the exact relationship between the junction temperature and the wavelength emitted is determined. The temperature of the laser diode is measured, at which the laser diode is outputting a beam having the critical wavelength where the laser emission is no longer considered to be eye safe. Since the value of dX / dT is known for such junctions, the value of the predetermined temperature T1 used by the controller to limit laser emission, can be adjusted by the controller to match the temperature measured in the preliminary calibration procedure, which corresponds to the critical wavelength at which the laser emission is no longer considered eye safe. Alternatively diodes may be sorted, before assembly into the laser system, based on the temperature at which the critical wavelength is emitted, and assigned a different temperature T1 for each “bin” of diodes.

[0061] The diodes having a median critical wavelength are assigned a first predetermined temperature Tl, which is encoded into the settings of the controllers in the transmitters in which they will be installed. Diodes having a shorter critical wavelength are assigned a higher predetermined temperature Tl, that too will be uploaded to the transmitters in which they will be installed. Such systems may be more problematic for general use, since they may take a long time to wake up in cold weather. Diodes with long values of critical wavelength, or very short values of critical wavelength may be problematic for use in the laser wireless power transmitters of the present application.

[0062] It is to be understood that this calibration process for the laser diode of each individual wireless optical power transmitter system, is not an obligatory process, but is dependent on the eyedanger of the system involved. Thus for systems using lower powered laser beams, where the danger of eye damage is greatly reduced, it is possible to forego this additional calibration adjustment, while for higher power laser emissions, this calibration step takes on greater importance. In any event, use of the term “predetermined temperature Tl”, both in the description, and as claimed, is understood to include both directly used measurements of the temperature sensor, or a controller-adjusted value of the measurement of the temperature sensor. Reference is now made to Fig. 3, which illustrates an exemplary flow chart showing one method by which the control system of the presently described system operates from start-up, while maintaining safe operation of the system, based on measurements of the laser diode temperature by sensor 103.

[0063] In step 301, the system is turned on, and in steps 302 and 303, the system performs self-test procedures, such as a self-test of the temperature sensor or a self-test of the laser diode itself. Step 302 is especially relevant to the safe operational procedure of the currently described system. Should there be an indication of the temperature sensor being disconnected or short- circuited, as may be evident by a lack of voltage or an excess voltage on the sensor, the system should go into an error state, such as terminating laser emission, or at least, switching to a lower safe emission level.

[0064] If the system passes the self-tests of steps 302 and 303, then in step 304, a first temperature measurement is performed, the example shown in Fig. 1 showing the common case when the temperature sensor 103 measures the heat sink temperature. At this stage, the system may not know if the laser diode temperature is higher or lower than the heat sink temperature, since the immediate “history” of the system operation may not be known. Thus, for instance, if the system start-up is after a reboot of the controller or of the entire system, the laser diode may be close to its normal operating temperature, whereas if the measurements are being made after a cold start-up, the laser diode junction may be at a significantly lower temperature. In some implementations, a short delay time may thus be implemented before determining the temperature of the laser diode, this delay allowing the heat sink and the laser diode to move closer to thermal equilibrium.

[0065] In step 305, if the measured temperature is higher than the first threshold temperature, Tl, indicative of an “eye-safe” operational wavelength above the lower wavelength limits at which lasing can occur at 1307nm, 1305.7nm, 1305.2 nm 1305nm, 1300nm, or 1298nm, then in step 309, the laser may be turned on immediately, but only at a low power setting. This is a safety feature, since it may be unsafe to turn the laser power on at its high level until it is determined in which direction the temperature is moving, i.e. whether the laser diode is cooling down, which is potentially in a less safe direction for the temperature to be moving, or is heating up, which is a safer direction for the temperature to be moving. Once the laser is turned on at low power, the laser diode junction heats up, and a temperature difference develops between the junction, and the temperature sensor, which is usually attached to the heat sink outside of the laser diode housing. The decision to allow transmission of higher power by the transmitter will be determined later in step 311.

[0066] If, on the other hand, the temperature is at or below that first threshold Tl, which is a temperature region indicating a potentially eye damaging wavelength, the controller does not yet allow the laser to be turned on, but instructs the beginning of heat generation in step 306. In step 307, monitoring of the temperature is continued to determine whether the temperature has reached a second threshold temperature T2 threshold, which is somewhat higher than Tl, by a predetermined amount. The heat generator remains in operation so long as the monitored temperature has not reached T2 or higher. If the temperature does reach T2 or higher, then the heating generator may be turned off or at least reduced in step 308, and the laser diode can be turned on in step 309, but as previously mentioned, only at low power. The reason for a second threshold T2, higher than Tl is to ensure that the heating has affirmatively raised the temperature of the laser diode above the first threshold Tl, rather than just reaching Tl. The use of this second threshold temperature T2 provides assurance that the temperature difference between the laser diode and the temperature sensor, generally on the heatsink, is positive, meaning that the diode is at a higher temperature than the temperature sensor. This determination shows that the diode temperature is moving in the desired direction, that of heating up, thereby indicating that the emitted wavelength is moving to a safe region for eye exposure. On the other hand, if the laser diode were not heating up for any reason, such as defective heating or a very low ambient temperature, T2 would not be reached, and the controller does not enable laser emission.

[0067] In step 310, temperature monitoring is continued at close intervals, or even continuously, with the object of ensuring that the expected rise in temperature as a result of beginning laser operation, even if only at a low power level, has resulted in the rise of temperature above the threshold Tl. This criterion is assured by checking that the temperature has reached, and continues to remain, above T2, which is somewhat above temperature Tl, and is not falling back again into the potentially eye-unsafe region below Tl. If the temperature remains above T2 for a predetermined time T, this indicates that some level of temperature equilibrium between the heatsink and the junction has been reached, and that the laser temperature is remaining stably above T2 before commencing high power operation, and is not falling again. If the temperature is below T2, the laser continues to operate at low power, continuing to generate heat as it does that, until the temperature is high enough to allow high power operation. The predetermined time T may be dependent on either of the speed of response of the laser diode and any intervening heat capacitive or heat conductive bulk between the laser diode itself and the temperature sensor, or on the rate of change of temperature while the temperature was climbing to reach either the T1 threshold or the T2 threshold.

[0068] As a consequence of the temperature remaining above T2, in step 311 the laser power is allowed to be raised to its high power mode. Once the laser is operating at high power, it is likely that the internally generated heat will cause its temperature to increase more definitely, thus moving even further into the “eye-safe” wavelength region. However, to ensure uncompromised safety, the temperature monitoring and stabilization is continued, to ensure that the system remains in the “body safe” region clearly above Tl, as assured by maintaining a temperature of at least T2, even if conditions occur that could make the temperature of the laser fall below Tl, even when outputting the high power level. These steps will now be described in steps 312 to 316.

[0069] During high power operation, the system determines in step 312 whether the temperature monitored has risen to more than a third threshold T3, which is higher than T2. This determination is not required from the eye-safety aspect, since, even though such danger becomes less as the wavelength increases, there would be a subjective warning of potential danger to the human’s eyes, because of the above-described “body safe” effects of the laser beam. However, at wavelengths above those corresponding to emission at temperature threshold T3, the increased laser absorption arising from water vapor in the air, would result in a reduction of the efficiency of power transmission. Therefore, if the temperature of the laser diode rises above threshold T3, then in step 313, a cooling system (not shown in Fig. 2 since such a cooling system may be ancillary to the heat sink) may be actuated to return the laser diode temperature to below T3, at which point the normal temperature monitoring of step 314 is continued, without disturbing the high power supply of laser power to a remote receiver.

[0070] Temperature monitoring is continued in steps 314 and 315, and so long as the temperature does not, at any point during operation, drop below a fourth threshold, T4, lower than Tl by a predetermined amount, high power mode operation of the laser can continue in step 311. However, if the temperature does fall below T4, high power lasing must be terminated, since temperatures below Tl, and even more so, below T4, represent a wavelength region of danger to the eye. The reason why the threshold temperature T4 is defined as being below Tl by a predetermined amount, lies in the observation that during laser emission in the high power mode, the temperature of the junction will be higher than the temperature measured by the sensor 103. Consequently, the temperature threshold T4, at which the wavelength is determined to be unsafe for eye exposure, is measured by the sensor, which is at a lower temperature than the actual temperature at the laser diode junction. Since the laser junction is the point which actually determines the wavelength emitted by the laser, the measured threshold T4 must be less than Tl. In the simplest control routine, unlike that shown in Fig. 3, when T4 is reached, the laser emission is terminated and the system returns to step 305. However, since such a drastic response may cause a greater disruption to the supply of power from the laser than may be necessary, according to a more complex operational routine, as shown in the embodiment of Fig.3, the status to which the control returns the system can optionally be made to depend on the rate of fall of temperature, dT / dt, which the laser is experiencing, as determined in step 316.

[0071] If the rate of fall of the temperature dT / dt, measured in step 316 is above a predetermined rate, R, this is regarded as a system error state, and the controller terminates lasing at step 317, and returns the system operation to step 305. This is an important safety step, since such an elevated rate of temperature change may indicate a control fault, or a sensor fault, a laser fault, a heatsink fault, or another circuit fault, any of which could indicate an uncontrolled entry into an unsafe situation, mandating immediate cessation of lasing until the problem is determined. On the other hand, if the temperature fall is more gradual, at less than the predetermined rate, R, the control may proceed just to step 309, reducing the laser power to its safe level, since the temperature fall may just be a minor circumstantial situation or an environmental change, or a transient increase in the heatsink cooling effect such as could be caused by a sudden increase in speed of the fan, or of the cooling water, or any other incidental effect, and does not indicate a system fault. Under those conditions, laser operation back at high power should be speedily restored, as determined in steps 310 and 311.

[0072] The above described method of Fig. 3, describes the operation of a system intended to provide an optimally high level of eye safety, since it includes operational steps which take into account two novel control featurees:

[0073] (i) the possible existence of the different temperature of the junction of the chip itself, from that of the heat sink or the housing of the laser diode chip, which is where the temperature is actually measured, and

[0074] (ii) the dynamics of the temperature changes occuring in the laser diode, namely, not only the extent of this temperature difference, but also the direction of the temperature difference, meaning whether the laser diode junction is assumed to be hotter than the heatsink surface, or cooler than the heatsink surface. This is dependent on the dynamics of the power changes in the laser diode emission.

[0075] However, it is to be understood that there may be systems having laser diodes with packaging or housing techniques, which provide a high level of thermal conductivity between the diode chip itself and the outer surface of the housing, which is where the temperature measurement may be made. When changes in the operation of the system occur, such laser diodes then display a speedy attainment of temperature equilibrium between the junction and the temperature sensor. If the time taken to achieve thermal equilibrium is of the same order as the safe exposure time of the eye to the laser beam being emitted, then the neeed for a method that also takes into account the dynamics of the temperature changes may be uneccesary. The operational algorithm used to control the temperature dependent wavelenth of the laser diode emission may then be simplified, such that only one low temperature threshhold needs to be used to provide human protection when the laser diode temperature falls to a temperature that is associated with a wavelength that may be hazardous to the eye, and another, upper threshold temperature, needs to be used to ensure that the temperature of the laser diode does not rise to a temperature that is associated with a wavelength that may show an unacceptable level of absorption from the water vapor in the air through which the beam is being transmitted.

[0076] Additionally, even in systems using laser diodes not having speedy attainment of temperature equilibrium, if the changes in power performed are relatively small, the same criteria as those presented above apply, and a simpler method of protection may be applied to the laser diode system. This would then be based on the exemplary method shown in Fig. 3, but with additional steps which calculate the rate of change of laser power, and if below a predetermined level, do not require separate levels of the low and high temperature thresholds associated with the dynamics of the power, and hence temperature, changes.

[0077] Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. Furthermore, it is appreciated by humans skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a human of skill in the art upon reading the above description and which are not in the prior art.

Claims

CLAIMSWe claim:

1. A transmitter for providing eye-safe wireless optical power transmission, comprising: a laser diode configured to emit a beam in the near infra-red region, the wavelength of the beam decreasing with decrease of the temperature of the junction of the laser diode; a temperature sensor attached to the laser diode, such that it provides an indication of the junction temperature of the laser diode; and a controller, receiving an input from the temperature sensor, and adapted to control the power output of the laser diode in accordance with the temperature measured, wherein the controller is configured to limit laser emission of the laser diode when the temperature measured by the temperature sensor is below a first predetermined temperature Tl, below which the wavelength of the laser emission is not considered to be eye-safe.

2. A transmitter for wireless optical power transmission according to claim 1. wherein limitation of the laser emission comprises either a reduction of beam power to an eye-safe level, or cessation of lasing.

3. A transmitter for wireless optical power transmission according to either of the previous claims, wherein when the temperature measured by the temperature sensor is below the first predetermined temperature, Tl, the controller is further configured to enable activation of heat generation to increase the temperature measured to a second predetermined temperature T2 above the first predetermined temperature Tl.

4. A transmitter for wireless optical power transmission according to claim 3, further comprising a heat generator to provide the heat generation.

5. A transmitter for wireless optical power transmission according to claim 3, wherein the heat generation is achieved by internal heating by electronic activity of digital circuitry associated with the transmitter.

6. A transmitter for wireless optical power transmission according any of the previous claims, wherein the first predetermined temperature T1 corresponds to a temperature of the laser junction at which the emitted laser beam has a wavelength less than the wavelength at which the time taken for sensory impact to be felt, of the impingement of the laser beam on a tissue of a human, is considered to be longer than the time taken for the laser beam to cause damage to the eye of the human.

7. A transmitter for wireless optical power transmission according to any of claims 1 to 5, wherein the first predetermined temperature T1 corresponds to a temperature of the laser junction at which the emitted laser beam has a wavelength less than a wavelength at which it is determined that the time taken from exposure to the laser beam until the beam is felt on a tissue of a human, is longer than the maximum time duration during which it is considered safe to expose an eye of the human to the laser beam.

8. A transmitter for wireless optical power transmission according to any of the previous claims, wherein the controller is configured to operate the transmitter at a high power mode only after a predetermined time during which it has operated in a low power mode.

9. A transmitter for wireless optical power transmission according to any of the previous claims, wherein the controller is further configured to reduce the power output of the laser diode to a predetermined reduced level, when the temperature reported by the temperature sensor rises above a third predetermined temperature T3.

10. A transmitter for wireless optical power transmission according to claim 9, wherein the third predetermined temperature T3 corresponds to a temperature of the laser junction, at which the emitted laser beam has a wavelength above which the beam has an absorption by water vapor greater than a predetermined level.

11. A transmitter for wireless optical power transmission according to claim 10, wherein the predetermined level of beam absorption causes a reduction in the transmission efficiency of the laser beam greater than a predetermined level.

12. A transmitter for wireless optical power transmission according to any of claims 1 to 8, further comprising a cooling system, and wherein the controller is configured that when the temperature reported by the temperature sensor rises above the third predetermined temperature T3, the cooling system is activated to reduce the temperature of the laser diode to at least below the third predetermined temperature T3.

13. A transmitter for wireless optical power transmission according to claim 12, wherein the third predetermined temperature T3 corresponds to a wavelength of the laser above which it is determined that absorption of the laser beam by water vapor in the transmitted beam path is such as to cause a reduction in transmission efficiency of the beam to a receiver, greater than a predetermined level.

14. A transmitter for wireless optical power transmission according to any of the previous claims, wherein, during operation in the high power mode, the controller is configured to terminate lasing when a fall of temperature at greater than a predetermined rate is detected, to below a fourth predetermined temperature T4, the fourth temperature T3 being less than the first predetermined temperature Tl.

15. A transmitter for wireless optical power transmission according to any of claims 1 to 13, wherein, during operation in the high power mode, the controller is configured to reduce the system operation to the low power mode when a fall to below a fourth predetermined temperature T4 is detected at less than a predetermined rate, the fourth temperature T4 being less than the first predetermined temperature Tl.

16. A transmitter for wireless optical power transmission according to any of the previous claims, wherein the controller is configured to limit laser emission only when there is a loss of power exceeding a predetermined level, between the power of the transmitted laser beam and the power of the laser beam received at a remote location.

17. A method for providing eye- safe transmission of laser power to at least one receiver, comprising the steps of:providing a laser power transmitter having a laser diode configured to emit a beam in the near infra-red region, the wavelength of the beam decreasing with decrease of the temperature of the laser diode; using a temperature sensor attached to the laser diode to measure a temperature providing an indication of the junction temperature of the laser diode; controlling the power output of the laser diode in accordance with the temperature measured; and limiting emission of the laser diode when the measured temperature is below a first predetermined temperature Tl, below which the wavelength of the laser emission is not considered to be eye-safe.

18. A method for providing eye- safe transmission of laser power to at least one receiver according to claim 17, wherein the emission of the laser diode is limited by reducing the beam power to an eye-safe level, or by terminating the power output of the laser diode.

19. A method for providing eye- safe transmission of laser power to at least one receiver according to either of claims 17 and 18, further comprising the step of activating heat generation to increase the temperature measured to a second predetermined temperature T2 above the first predetermined temperature Tl, if the temperature measured by the temperature sensor is below the first predetermined temperature Tl.

20. A method for providing eye-safe transmission of laser power to at least one receiver according to claim 19, wherein the heat generation is performed by use of a heat generator.

21. A method for providing eye-safe transmission of laser power to at least one receiver according to claim 19, wherein the heat generation is achieved by internal heating by electronic activity of digital circuitry associated with the transmitter.

22. A method for providing eye- safe transmission of laser power to at least one receiver according any of claims 17 to 21, wherein the first predetermined temperature Tl corresponds to a temperature of the laser junction at which the emitted laser beam has a wavelength less than the wavelength at which the time taken for sensory impact to be felt, of the impingement of the laser beam on a tissue of a human, is considered to be longer than the time taken for the laser beam to cause damage to the eye of the human.

23. A method for providing eye-safe transmission of laser power to at least one receiver according to any of claims 17 to 21, wherein the first predetermined temperature corresponds to a temperature of the laser junction at which the emitted laser beam has a wavelength less than a wavelength at which it is determined that the time taken from exposure to the laser beam until the beam is felt on a tissue of a human, is longer than the maximum time duration during which it is considered safe to expose an eye of the human to the laser beam.

24. A method for providing eye- safe transmission of laser power to at least one receiver according to any of claims 17 to 21, wherein the controller is enabled to operate the transmitter at a high power mode only after a predetermined time during which it has operated in a low power mode.

25. A method for providing eye-safe transmission of laser power to at least one receiver according to any of claims 17 to 24, wherein when the temperature reported by the temperature sensor rises above a third predetermined temperature T3, the controller is activated to reduce the power output of the laser diode to a predetermined reduced level.

26. A method for providing eye- safe transmission of laser power to at least one receiver according to claim 25, wherein the third predetermined temperature T3 corresponds to a temperature of the laser junction, at which the emitted laser beam has a wavelength above which the beam has an absorption by water vapor greater than a predetermined level.

27. A method for providing eye- safe transmission of laser power to at least one receiver according to claim 26, wherein the predetermined level of beam absorption causes a reduction in the transmission efficiency of the laser beam greater than a predetermined level.

28. A method for providing eye-safe transmission of laser power to at least one receiver according to any of claims 17 to 27, further comprising the step of activating a cooling system to reduce the temperature of the laser diode to at least below the third predetermined temperature T3, when the temperature reported by the temperature sensor rises above the third predetermined temperature T3.

29. A method for providing eye- safe transmission of laser power to at least one receiver according to claim 28, wherein the third predetermined temperature T3 corresponds to a wavelength of the laser above which it is determined that absorption of the laser beam by water vapor in the transmitted beam path is such as to cause a reduction in transmission efficiency of the beam to a receiver, greater than a predetermined level.

30. A method for providing eye-safe transmission of laser power to at least one receiver according to any of claims 17 to 29, wherein, during operation in the high power mode, if a fall of temperature at greater than a predetermined rate is detected, to below a fourth predetermined temperature T4, the fourth temperature threshold being less than the first predetermined temperature Tl, further comprising the step of terminating lasing during operation in the high power mode,31. A method for providing eye- safe transmission of laser power to at least one receiver according to any of claims 17 to 29, wherein, during operation in the high power mode, if a fall to below a fourth predetermined temperature T4 is detected at less than a predetermined rate, the fourth predetermined temperature being less than the first predetermined temperature Tl, further comprising the step of reducing the system operation to the low power mode.

32. A method for providing eye-safe transmission of laser power to at least one receiver according to any of claims 17 to 31, wherein the controller limits laser emission only when there is a loss of power exceeding a predetermined level, between the power of the transmitted laser beam and the power of the laser beam received at a remote location.

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